Several years ago, in 2009 and 2010, a string of unexplained floods and unusually high tides struck the East Coast. There was no easy explanation. No hurricane. No winter storm. But the waters kept spilling across the shoreline, from North Carolina’s Cape Hatteras to Canada.

The cause of that phenomenon may now have finally been found. Sea levels from New York to Newfoundland were undergoing an “extreme” surge unlike any other in recorded history, according to a new study in Nature Communications published this week. Calling the phenomenon “unprecedented” and “very unusual,” oceans along the East Coast rose roughly four inches between 2009 and 2010 in a rapid spike researchers compared to a “1-in-850-year event.”

“This is a very extreme event,” Jianjun Yin of the University of Arizona told The Washington Post in a phone interview. “The sea level has since dropped after that spike, but it is still much higher than it was when the spike began in 2009. … Global warming definitely contributed to this event.”

But global warming wasn’t the sole culprit. The study suggested a change in ocean currents coupled with persistent winds that pushed water into the region caused the spike — which may be the first of many. The seas are rising, Yin told The Post, but the ascent isn’t smooth or even. The mechanics of sea rise aren’t dissimilar to temperatures rising during spring. During some years, the weather gets warmer faster — but the general trend is upward.

“We should expect more of these extreme rises in sea level,” lead author Paul Goddard said. “And for this to happen more in the future.”

Yin has long studied how ocean currents affect sea levels. He developed a hypothesis involving a major current in the Atlantic Ocean called the Atlantic meridional overturning circulation. He said a slowing of that current would lead to higher tides along the East Coast. The current transports warmer water from the tropics to the waters of the North Atlantic and polar regions. Once there, the water cools, sinks, and is then transported back to the tropics through the deep ocean in a system that works like a conveyor belt. As LiveScience explained, “the water temperature differences drive the current.”

But then in 2009 and 2010, a wrench was thrown into the system with warmer-than-average water temperatures in the North Atlantic’s Labrador Sea. Recent research, the scientists noted, showed the warm water perhaps decreased the strength of the current by as much as 30 percent. “The anomalous heat made the surface waters less dense and less likely to sink, and it created a bottleneck” of water along the Eastern Seaboard, Goddard told LiveScience.

At least that was the thinking. But who could be sure? So two years ago, Yin dispatched Goddard, his doctoral student, to look into it. Had the seas actually risen?

Goddard set out to compile data from all of the tide gauges lining North America’s East Coast, from Key West, Fla., to Newfoundland. The 40 gauges had monitored the seas since 1920. Then when Goddard came back with the data, there it was: a jump in sea levels “sticking out like a sore thumb,” he said. Beginning in 2009, the seas in the Northeast had surged around four inches.

“The levels we saw were the highest on record — and the highest by far,” Goddard told The Post. “This was a remarkable outlier.” He added in a statement: “To me, it was like putting together a puzzle.”

But when the pieces fit together, the news wasn’t good. Rowan Sutton, a climate scientist with the University of Reading in the United Kingdom, told the BBC that climate models suggest there will be more of these spikes. “This study identifies a record-breaking high sea level event. … There is strong evidence that the likelihood of such events has been increased by climate change, and we should expect more such events in the future.”

These predictions are especially ominous for the East Coast, Dan Hodson of the University of Reading told the BBC. The study shows how global warming can alter sea currents, which can result in higher sea levels. And some of some of the region’s most active sea currents are right along the Eastern Seaboard. “The East Coast of North America is quite close to an area of active, fast ocean currents,” Hodson said. “And so it is quite sensitive to changing ocean circulation.”

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